1. Field of the Invention
The present invention relates to a display device employing an electron beam for erasing a spot afterglow on a video tube for use in apparatus such as a television receiver, a monitor, or the like, and a method of erasing a display screen thereof.
2. Description of the Related Art
When a television receiver is operated, the television receiver modulates, based on R, G, B video signals, an electron beam radiated from a cathode disposed in an electron gun of a cathode-ray tube (CRT) that lands on an anode on a fluorescent screen applied with a high voltage. Thus, the television receiver displays a color picture on the CRT.
Therefore, a high voltage (hereinafter referred to as an anode voltage) applied to the anode on the fluorescent screen in the CRT when the television receiver is operated remains for a certain time even after the television receiver has been de-energized. As a result, there occurs an afterglow phenomenon in which, when a deflection scanning is stopped, the electron beam converges on a substantial center on the picture screen of the CRT and persistent light spot remains. This phenomenon leads to screen burning of the CRT.
However, a Trinitron (registered trademark) CRT employing a single-electron gun includes an incorporated resistor, i.e., an inner bleeder resistor (IBR) in order to apply to an electrostatic deflection plate in the CRT a convergence voltage which is 4% to 8% smaller as compared with the anode voltage. In this case, when the television receiver is de-energized, the residual anode voltage on the fluorescent screen is quickly discharged to thereby avoid the spot afterglow to a certain degree.
FIG. 1 is a cross-sectional view showing a part of an arrangement of a single-electron gun type CRT including the IBR. An anode button 51 has a shape of a cavity cap. The anode button 51 is applied with an anode voltage from a high voltage generator (not shown) provided in the television receiver. An internal carbon 52 is coated on the inner surface of a CRT 50. The internal carbon 52 supplies the anode voltage applied from the anode button 51 to a fluorescent screen (not shown) in the CRT 50 and to an IBR 54 of an electron gun 53.
The IBR 54 divides the anode voltage to apply a convergence voltage to an electrostatic deflection plate 55. The convergence voltage is adjusted by changing a resistance value of a variable resistor 56 provided outside the CRT 50.
FIG. 2 shows a diagram showing an example of a structure of the IBR 54 which is the incorporated resistor. A resistor 62 in the form of a pattern is disposed on a ceramic substrate 61. The IBR 54 includes an electrode 63 applied with the anode voltage, an electrode 64 for applying the convergence voltage to the electrostatic deflection plate 55, and an electrode 65 connected through the variable resistor 56 to a ground line. An overcoat glass 66 is made of an insulating material and formed on the ceramic substrate 61 except at the electrodes 63, 64 and 65 so as to cover the resistor 62.
Thus, when the television receiver having the IBR 54 incorporated in the CRT 50 is de-energized, the anode voltage remaining on the fluorescent screen of the CRT 50 is quickly discharged therefrom through the internal carbon 52, the IBR 54, and the variable resistor 56, preventing the spot afterglow on the fluorescent screen of the CRT 50.
When a Trinitron CRT employing three electron guns and a shadow mask type CRT employing three electron guns is used, three IBRs must be provided inside the respective three electron guns in order to prevent the spot afterglow. Therefore, there is then the disadvantage that the CRT inevitably becomes large in size and it costs more to manufacture such CRT.
It is very difficult to dispose the three IBRs in a small space in the CRT employing three electron guns. After the television receiver is de-energized, the high voltage remains on the anode surface for a certain time. Especially, when an aperture grill-type CRT employing three electron guns and having a high ratio of apertures relative to a screen is used, there is then the problem that the residual voltage generates the spot afterglow and the spot afterglow damages the fluorescent screen.